Buck-boost converters may convert an input direct current (DC) voltage to a higher or lower output DC voltage. Buck-boost converters may operate with a buck or step-down functionality wherein buck legs or subcircuits are in operation. Buck-boost converters may operate with a boost or step-up functionality, wherein boost legs or subcircuits are in operation. Furthermore, buck-boost converters may operate with a buck-boost functionality, wherein buck legs and boost legs are both in operation at the same time, and the converter may convert DC voltage to a higher or a lower voltage. Buck-boost converters may be made with an inverting topology or a non-inverting topology. In an inverting topology, a buck-boost converter may produce output voltage that has an opposite polarity as the input voltage to the buck-boost converter. In a non-inverting topology, a buck-boost converter may produce output voltage that has a same polarity as the input voltage to the buck-boost converter.
In order to accomplish a buck-boost converter that may operate with buck, boost, and buck-boost functionality, the buck-boost converter may be implemented in a four-switch topology. The four-switch topology may enable operation of the converter alternately in buck, boost, and buck-boost mode.
The transition between buck, boost, and buck-boost functionality may be triggered or handled by a forced mode transition approach. This approach utilizes comparators for monitoring the input voltage and partitioning the input voltage range into zones from lowest to highest input voltage, one each for boost, buck-boost, and buck operation. As input voltage reaches a different range, different parts of the circuit may be switched on and off. This approach may have bad output performance during transitions between boost, buck-boost, and buck operation. Voltage drop and overshoot occur as the converter transitions from one operation to another.
The transition between buck, boost, and buck-boost functionality may be triggered or handled by an automatic mode transition using voltage mode control. This approach utilizes an offset voltage added to a ramp voltage to generate two separate predefined voltage ramps. The separated ramp voltages are in turn compared with the output of an error amplifier. The error amplifier compares the output of the converter with a defined, target value. The output of the error amplifier may generate pulse width modulation (PWM) signals. However, this approach is limited to voltage mode-controlled converters. This causes drawbacks such as a slow response to changes in the load. Furthermore, such a converter with continuous conduction would be difficult to construct with regards to compensation of the converter under all operation conditions, as the output capacitor and inductor already form a second order system.
Another approach may include automatic mode transition with peak current mode control, which uses two current ramps and compares these ramps with an output of a voltage-loop compensator to generate PWM signals. One of the current ramps is generated by sensing an inductor current and the other current ramp is generated by adding a pedestal to the first current ramp. This approach has drawbacks. It is very difficult to define an appropriate pedestal voltage to achieve a perfect matching between the two current ramps with a minimum of overlap and no gaps between the ramps. To maintain correct regulation, there should preferably be no gap between buck and boost operation regions. To ensure this, both regions may overlap somewhat to account for possible component tolerances. However, overlap causes an efficiency penalty due to increased switching losses in the buck-boost region, as all four switches are switched during one cycle. Therefore, the overlap or buck-boost region should preferably not be larger than needed. Due to the non-ideality of components, such as capacitors and inductors, there are times when the current ramps overlap but there are also times when the current ramps separate. The larger the overlap, the more that the converter will work in buck-boost operation, reducing efficiency. The larger the separation, the more the converter will operate in a mode in which regulation is not working correctly, wherein the output of the converter is not regulated but defined by the load of the converter.
Yet another approach may include automatic mode transition with average current mode control. This approach uses two artificial current ramps which are compared in turn to the output of an average current-loop compensator to generate PWM signals. Like in automatic mode transition with peak current mode control, one ramp is superimposed over another ramp. However, this approach may require a complicated compensation network. When targeting large ranges for input, output, and load, stable operation of the converter under all conditions is very difficult.
The disadvantages of various converters described above have been recognized and identified by inventors of embodiments of the present disclosure. Embodiments of the present disclosure may provide converters with automatic mode transition that solve at least some of such identified problems, such as bad performance between boost, buck-boost, and buck operation, slow response to changes in a load of the converter, difficult implementation of compensation under various operating conditions, misalignment of current ramps, a complicated compensation network, and stable operation over large ranges of input, output, and load values.
Inventors of embodiments of the present disclosure have recognized that four-switch, single inductor buck-boost converters are challenging to control. Embodiments of the present disclosure include methods and circuits for controlling a four-switch, single inductor buck-boost converter with automatic, smooth, and better mode transition without sacrificing converter performance regarding efficiency, bandwidth, input-output range etc. Bad mode transition and bad control method degrades converter performance and limits applications of the topology. Embodiments of the present disclosure may include improvements over existing control techniques, such as better mode transition performance, better regulation loop in the various operation modes, higher noise immunity, both smooth and automatic mode transitions, higher efficiency at light load, and may be suitable for power conversion applications having large input and output voltage swing and need both buck and boost functions.